Reduction is defined as the transfer of electrons between different chemical species as part of a chemical reaction. The species that receives the electrons is ‘reduced’ because of the decrease in the oxidation state of the element or group of atoms involved. The chemical species (element or group of atoms) that donates the electrons is oxidised in the process.
A chemical species that readily donates its electrons is called a reducing agent. Meanwhile, a chemical species that readily accepts electrons is called an oxidising agent. The complete balanced reaction is called an oxidation-reduction reaction, or a ‘redox’ reaction for short.
Continue reading to learn how and why electrons are either attracted or readily donated by certain chemical species, and more about some of the common applications of reduction reaction in industries and in everyday life.
In this post:
Understanding Reduction in Chemistry
To understand reduction reaction, and by extension, oxidation reaction, in chemistry, we must first understand the concept of oxidation states. The oxidation state is the number of electrons a chemical species can gain, lose (donate), or share. Some elements have multiple oxidation states. These are commonly metallic elements.
A reduction reaction occurs when a chemical species gains electrons, while an oxidation reaction occurs as another chemical species loses electrons.
An example of this is in rusting nails, which involves both oxidation and reduction reactions.
Rust is iron oxide, which is formed by the reaction of oxygen in the air with the iron atoms of the nail as facilitated by water or moisture acting as catalyst. Rust is composed of iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO(OH), Fe(OH)3). The iron atoms undergo oxidation while the oxygen atoms undergo reduction.
How Reduction Works at the Molecular Level
At the molecular level, a chemical species, which can be atoms (element) or groups of atoms, can transfer electrons to other atoms or groups of atoms of different substances.
The chemical species that lost its electrons is said to be oxidised. Conversely, the chemical species that gained electrons is said to be reduced. New chemical bonds are formed in the process.
The Role of Electrons in Reduction
Just like in all other chemical reactions, the electrons are the main players. Bonds are broken and made with the exchange, sharing, and transfer of electrons. New substances are formed in the process.
However, chemical reactions are not totally random, and there are also some fundamental rules that chemical reactions follow. Reduction is a distinct type of reaction because it is inseparable from oxidation reaction.
If a chemical species is reduced, it necessarily means that another chemical species is oxidised in the process. One species donated (oxidised) electrons while another species gained electrons (reduced).
Electron Transfer in Reduction Reactions
Some substances (elements or compounds) readily accept electrons, while others readily donate electrons. These are reducing agents and oxidising agents, respectively.
Since reduction is inseparable from oxidation, they are typically simply called redox reactions. It could be any of the following chemical reactions: combustion, combination, decomposition, displacement, and disproportion.
This means that whenever the transfer of electrons occurs in reduction, bonds can either be broken or formed. For example, in a typical combustion reaction, new bonds are formed from breaking the bonds of organic compounds. Oxygen combines with the carbon of an organic compound to form carbon dioxide and water.
Identifying Reduction in Redox Reactions
Reduction is the half reaction of redox reactions. It represents the gaining of electrons – it’s the side of the chemical equation that is negatively charged.
It can either be spontaneous, such as in the case of voltaic (galvanic) batteries, or it can be nonspontaneous, such as in the case of electrolytic cells.
Oxidation vs. Reduction
Both oxidation and reduction are half reactions of redox. They are inseparable because they are complementary reactions. When oxidation occurs in one chemical species, reduction necessarily occurs in another chemical species. Redox reactions can either be spontaneous or nonspontaneous.
Many redox reactions, such as fuel combustion, rusting, and bleaching, are spontaneous reactions. This means that they do not require energy input to occur. Non-spontaneous redox reactions require energy inputs to occur.
Comparing Oxidation and Reduction
Oxidation and reduction are complementary and co-dependent reactions. One cannot occur without the other. Nonetheless, they have differences, chief of which is the question of which side donates and which side receives electrons during a chemical reaction.
Another difference, as a consequence of the electron transfer, is the relative charge changes of the half reactions. The oxidation state increases in oxidation, while the opposite is true for reduction.
Examples Demonstrating Oxidation and Reduction
One good example of redox reaction is the one that occurs in voltaic (galvanic) batteries. This is a type of spontaneous redox reaction that converts chemical energy into electrical energy. As the electrons flow from the anode to cathode, an electrical current is produced.
In this reaction, the zinc electrode is oxidised while the copper electrode is reduced. The electrical potential is consumed when an equilibrium of ions is reached.
Another example of redox reaction is the electrolysis of water. This is a type of nonspontaneous redox decomposition reaction. You need energy input in the form of electricity (direct current) to induce redox reaction. The water is split into its constituent elements in the process.
Oxidation occurs in the anode as four electrons, four hydrogen ions, and one diatomic molecule of oxygen are produced for every two molecules of water. Meanwhile, reduction occurs in the cathode as one diatomic molecule of hydrogen is released for every two hydrogen ions gaining two electrons.
Applications of Reduction Reactions
As previously mentioned, a reduction reaction is just a half reaction of the redox reaction. It always simultaneously occurs with oxidation. However, it does not necessarily mean that reduction reactions and oxidation reactions have exactly the same applications.
For example, even in gold electroplating, which is a nonspontaneous redox reaction, the gold ions migrate to the cathode (the site of reduction reaction) where they are neutralised and deposited.
Industrial Applications of Reduction
There is a wide range of industrial applications of reduction reactions (redox by extension). These include producing electrical energy through batteries, extracting metals, manufacturing consumer products like bleach, and treating municipal water.
Reduction in Everyday Chemical Processes
If you’re drinking tap water or even bottled water, you are benefiting from a reduction reaction.
Water mineral and salt impurities are removed from the water via a redox reaction by reducing the dissolved solids. This allows them to coagulate and settle down the bottom and be separated.
Meanwhile, exposed metal structures, such as bridges, are coated with zinc to protect them from corrosion. The zinc is oxidised while the oxygen is reduced. This provides protective layers of zinc oxides on the steel surfaces.
Summary
Reduction is inseparable from oxidation. Together they are called redox reactions. However, reduction itself has a separate role in many cases, such as in gold plating. Reduction reactions are used in many industrial processes, such as in manufacturing fertilisers, water treatment, and reducing air pollution from factories.
